How much horsepower can a 5.3 make naturally aspirated?

Stock: GM rated the Gen III/IV 5.3s at 270–320 hp depending on version and year, and the direct-injected Gen V 5.3s at 355–376 hp (see are all 5.3 blocks the same?).

With bolt-ons, no boost or nitrous — Speedway Motors' 5.3 recipes:

Target	Method	Parts
~400 hp	Cam swap	BTR Truck Norris cam (or NSR version for stock springs); BTR valve springs; GM balancer bolt; Fel-Pro timing cover gasket set
~450 hp	Cam + intake + throttle body	Edelbrock Rollin Thunder cam; PAC 1218 valve springs; PRW trunnion upgrade kit; Edelbrock LSX 92 mm intake; Holley Sniper 92 mm throttle body

Speedway notes the 400 hp cam-swap level is near the limit of the stock injectors and fuel system, and reports a junkyard LM7 5.3 making 430 hp with just a cam swap (BTR Truck Norris) in its C10 project.

Past ~450 hp: Speedway's 5.3 recipes move to nitrous and turbo (up to ~700 hp with valvetrain upgrades), and Speedway notes the stock LS fuel filter/regulator assembly tops out around 450 hp — beyond that, larger lines and an external regulator. See making power with an LS, size your fuel system with the injector & pump calculator, and compare cams in the cam finder.

Recipe power figures are Speedway's targets for those combinations; results vary with tune, heads, exhaust and how power is measured (crank vs wheel).